Crustaceans – From Krill to Lobsters

Crustaceans are a very big grouping of arthropods. Arthropods are invertebrate animals that have exoskeletons, jointed appendages and segmented bodies. This group of animals includes the familiar creatures like crabs, crayfish, lobsters, barnacles, krill and shrimp. The 50, 000 or so species of crustaceans range in size from the exceptionally small, such as Stygotantulus stocki at 0.1 millimeters, to the large, such as the Japanese spider crab, which features a leg span of almost 13 feet and a weight of almost 45 pounds. Crustaceans molt their exoskeletons in order to grow. Crustaceans are different from other kinds of arthropods because of the nauplius larvae and also the presence of two-part limbs.


A crustacean’s body is made up of various body segments, which are organized into three regions. The three regions are head or the cephalon, the abdomen or the pleon, and the thorax. A crustacean’s thorax and head may be joined together to create something called a cephalothorax, basically a body part that is totally distinct from the abdomen behind it. The body of the crustacean is shielded by the exoskeleton, which has to be molted for the animal to actually grow. Other parts of the crustacean’s exoskeleton can be fused together, too.

Every body segment, or somite, of the crustacean can have a pair of appendages. For example, the head has pairs of maxillae, mandibles and antennae; the thoracic body segments have legs, which can either be feeding legs or walking legs; and the abdomen has pleopods and also the telson. It has been speculated that the success of crustaceans in general has a lot to do with the quantity and the variety of the crustaceans’ appendages. Their appendages are usually split up into two distinct parts.

The main body of a crustacean is best termed an open circulatory system. In this specific system, a heart situated close to the dorsum is used to pump the blood haemocoel. The crustacean’s alimentary canal is comprised of a straight tube that features something like a gizzard, which is used to grind up the food the crustacean takes in. The crustacean’s kidneys are to be found near the antennae of the animal.


Most crustaceans are in fact aquatic, meaning they live in either freshwater or marine environments around the globe. Still, some kinds of crustaceans such as crabs, woodlice and hermit crabs have actually adapted to a life on dry land. It is said that crustaceans are as plentiful in the oceans as the insects are on dry land. Many crustaceans are called motile because they are able to move about independently. However, some crustaceans are parasitic and, as such, live their lives connected to their hosts. A few examples of these types of crustaceans are tongue worms, whale lice, fish lice and sea lice, all of which can be referred to as crustacean lice. Learn more about Krill Oil vs. Fish Oil.

Life Cycle

Mating system

Many crustaceans reproduce sexually and possess separate sexes. Remipedes and barnacles are examples of hermaphrodite crustaceans. It is not unheard of for some kinds of crustaceans to change their sex over the course of their lives. A phenomenon called parthenogenesis, which is when viable eggs are created by the female without the requirement of fertilization by a male, is widely common among crustaceans.


For many kinds of crustaceans, the fertilized eggs are just let go into the water column; still other kinds of crustaceans have come up with a mechanism for retaining the eggs until they are mature enough to be hatched. For example, many decapods carry their eggs connected to the pleopods, but plenty of isopods and notostracans create a brood pouch from the thoracic limbs and the carapace. Others like female Branchiura do not carry their eggs in ovisacs that are external, but, rather, attach them to rocks and other objects in rows. Many krill types actually carry their eggs in between their thoracic limbs, yet other kinds of crustaceans even have them connected in tangled strings of a considerable length.


Crustaceans go through a few different larval forms; the earliest of these is the nauplius. In many groups of crustaceans, there are additional larval stages, such as the zoea. Coming after the nauplius stage and preceding a crustacean’s post-larval stage, the zoea swim by using their thoracic appendages, which is different from nauplii. Nauplii use appendages for swimming. Megalopa, on the other hand, utilize abdominal appendages to swim, and megalopa also features spikes on its carapace, which can help these tiny organisms in directional swimming.


The word “crustacean” has a long history. The works of both Guillaume Rondelet and Pierre Belon use the word. The first legitimate work to use the name “Crustacea” was 1772’s Zoologia Fundamenta by Morten Thrane Brunnich. The Crustacea subphylum comprises about 52, 000 species, but the quantity of undescribed species may actually be considerably higher. In total, there are six classes of crustaceans: remipedia, ostracoda, malacostraca, maxillopoda, cephalocarida and branchiopoda.

  • Crustacean Society Website: This website promotes the further study of all factors in the crustacean’s biology.
  • Collection: This is the website for one of the biggest state collections of crustaceans in North America.
  • All About Crustacean Shellfish: This website features information on crabs, lobsters and shrimps.
  • Lesson Plan (PDF): This website features a crustacean lesson plan designed to target school kids.
  • Gene Article: This website presents an article that talks about how a crustacean is the animal with the highest number of genes.
  • A Look at Molting: This website features information and a video on crustaceans molting.
  • Features: This website examines the features that are common to crustaceans.
  • Heart Comparison: This website explains the difference in a human versus a crustacean heart.
  • What are Crustaceans?: This website looks at the basics of crustaceans.